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Award Ceremony Speech

By giving the Nobel Prize in Physics of this year to Sir John
Cockcroft, Director of the Atomic Energy Research Establishment
at Harwell, and Professor Ernest Walton of Dublin University, the
Swedish Academy of Sciences has rewarded a discovery which stands
out as a milestone in nuclear research.

At the beginning of this century, the study of the naturally
radioactive substances had shown that their property of emitting
radiation is connected with spontaneous transmutations of their
atoms. It appeared, however, to be beyond human power to
influence the course of these processes.

The radiation emitted by a radium source contains swiftly moving
and positively charged helium atoms. By investigating the way in
which these particles are deflected by other atoms, the great
nuclear scientist Rutherford found in
1911 that an atom has a positive nucleus which is very small
compared to the whole atom but contains most of its mass. Besides
the nucleus, the atom contains negative electrons, moving around
the nucleus.

Continuing these investigations, Rutherford was able in 1919 to
produce transmutations of atomic nuclei by bombarding nitrogen
with helium nuclei from a radium source. Some of the helium
nuclei had enough energy to overcome the repelling electric field
and to penetrate into the nitrogen nucleus, in those rare cases
when they struck such a nucleus. The nitrogen nucleus thereupon
turned into an oxygen nucleus, while a hydrogen nucleus was
emitted.

Thus it became possible by external means to transform nitrogen
into oxygen, i.e., to transmute one element into another.

However, only a very few nuclear transmutations could be produced
by these natural projectiles, the helium nuclei from radioactive
substances. In order to produce nuclear transmutations on a
larger scale, and thus obtain further insight into the structure
of atomic nuclei, a more powerful stream of projectiles was
needed.

Accordingly, the end of the 1920's saw investigations of the
possibility of accelerating charged particles to high energies,
with the ultimate aim of using these particles to produce nuclear
transmutations. This year's Nobel Laureates in Physics were the
first to succeed in this task, by their joint work at the
Cavendish
Laboratory in Cambridge, of which Rutherford was at that time
the director. In planning this work, they realized the importance
of certain contemporary theoretical studies by Gurney and Condon,
and by Gamow. This work had shown that, because of the wave
properties of matter, there is a certain probability for a
positively charged particle to penetrate into a nucleus even if,
according to ordinary mechanical concepts, the velocity of the
particle does not suffice to overcome the electric repulsion from
the nucleus. Cockcroft had emphasized that the conditions are
particularly favourable if hydrogen nuclei are used as
projectiles, and that an accelerating voltage of only a few
hundred thousand volts should suffice to give observable
transmutations of light elements.

The work of Cockcroft and Walton was a bold thrust forward into a
new domain of research. Great difficulties had to be overcome
before they were able to achieve their first successful
experiments at the beginning of 1932. By then, they had
constructed an apparatus which, by multiplication and
rectification of the voltage from a transformer, could produce a
nearly constant voltage of about six hundred thousand volts. They
had also constructed a discharge tube in which hydrogen nuclei
were accelerated. Causing these particles to strike a lithium
layer, Cockcroft and Walton observed that helium nuclei were
emitted from the lithium. Their interpretation of this phenomenon
was that a lithium nucleus into which a hydrogen nucleus has
penetrated breaks up into two helium nuclei, which are emitted
with high energy, in nearly opposite directions. This
interpretation was later fully confirmed.

Thus, for the first time, a nuclear transmutation was produced by
means entirely under human control.

In order to get a detectable transmutation of lithium, a voltage
of little more than one hundred thousand volts was required. The
number of transmutations rose quickly as the voltage was
increased. The corroboration obtained in this way for the theory
which Gamow and others had propounded, and which was referred to
above, was of great importance.

The analysis made by Cockcroft and Walton of the energy relations
in a transmutation is of particular interest, because a
verification was provided by this analysis for Einstein's law
concerning the equivalence of mass and energy. Energy is
liberated in the transmutation of lithium, because the total
kinetic energy of the helium nuclei produced is greater than that
of the original nuclei. According to Einstein's law, this gain in
energy must be paid for by a corresponding loss in the mass of
the atomic nuclei. This assertion was satisfactorily confirmed by
Cockcroft and Walton, experimental errors being taken into
consideration. Somewhat later, more exact investigations based on
the same principles gave a complete verification of Einstein's
law. Thus a powerful method was obtained for comparing masses of
atomic nuclei.

In subsequent work, Cockcroft and Walton investigated the
transmutations of many other atomic nuclei. Their techniques and
results remain a model for nuclear research. As projectiles, they
also used the nuclei of heavy hydrogen, which had then just been
discovered. As end products, several atomic nuclei were obtained
which had not been known previously. Following the discovery of
artificially radioactive elements, by Frédéric and
Irène Joliot-Curie, they found that such elements can
also be produced by irradiation with hydrogen nuclei.

The investigations of Cockcroft and Walton disclosed a new and
fertile domain of research, consisting of the study of nuclear
transmutations of various types.

Their discoveries initiated a period of rapid development in
nuclear physics. Besides the apparatus of Cockcroft and Walton,
the cyclotron constructed by Lawrence, and various other particle
accelerators played important roles. By its stimulation of new
theoretical and experimental advances, the work of Cockcroft and
Walton displayed its fundamental importance. Indeed, this work
may be said to have introduced a totally new epoch in nuclear
research.

Sir John Cockcroft, Professor Ernest
Walton. The great nuclear scientist Rutherford, with whose work
your discovery is closely connected, sometimes used to say: "it
is the first step that counts". This saying may be applied in the
truest sense to your discovery of the transmutations of atomic
nuclei by artificially accelerated particles. Indeed, this work
of yours opened up a new and fruitful field of research which was
eagerly seized upon by scientific workers the world over. It has
profoundly influenced the whole subsequent course of nuclear
physics. It has been of decisive importance for the achievement
of new insight into the properties of atomic nuclei, which could
not even have been dreamt of before. Your work thus stands out as
a landmark in the history of science.

On behalf of the Royal Swedish Academy of Sciences may I extend
to you our warmest congratulations. I now ask you to receive your
Nobel Prize from the hands of His Majesty the King.